To promote transmission efficiency, suppressed carrier, phase modulated techniques are now being widely used in communication links for digital data. With such techniques, a transmitted wave includes no carrier component and all of the energy is developed in the side bands. Digital data may be transmitted by modulating the carrier at .+-. 90.degree., to provide bi-phase modulation; alternatively, the carrier may be phase modulated in 90.degree. increments to provide quaternary phase digital modulation.
A disadvantage of many prior art phase demodulators is that they have a relatively restricted linear operating region, limited to approximately .+-. 60.degree. variation in phase modulation relative to the carrier phase. Hence, the prior art devices frequently cannot be utilized to provide an accurate replica of analog variations that produce large phase variations.